/Engine/Source/Runtime/SlateCore/Public/Widgets/SWindow.h
Adds content to draw on top of the entire window
The added overlay slot so that it can be configured and populated
[SOverlay::FScopedWidgetSlotArguments](API\Runtime\SlateCore\Widgets\SOverlay\FScopedWidgetSlotArguments) AddOverlaySlot ( const int32 ZOrder )